1. Field of the Invention
This invention relates to calcium phosphate compositions, including cements, pastes and slurries, and to the methods for making and using them. In particular, the invention relates to hydroxyapatite forming cements, pastes and slurries prepared from a set of precursors which includes tetracalcium phosphate.
2. Description of Related Art
It has been known for some time that hydroxyapatite materials have the basic properties of human bones and teeth. A considerable amount of research has been directed to the remineralization of incipient dental lesions by deposition of hydroxyapatite, Ca.sub.5 (PO.sub.4).sub.3 OH, on such lesions, so that the hydroxyapatite is incorporated into the dental structure at the point of lesion.
Remineralization of tooth enamel has been carried out experimentally both in vivo and in vitro. These studies have concentrated on the remineralizing properties of saliva and synthetic solutions supersaturated with respect to hydroxyapatite. Two articles that give a good overview of this research are Briner et al., "Significance of Enamel Remineralization", J. Dent. Res. 53:239-243 (1974); and "Silverstone, Remineralization Phenomena", Caries Res. 11 (Supp. 1): 59-84 (1977). Additional experimental work in the areas of remineralization of calcium phosphate biomaterials may be found in Gelhard et al, "Rehardening of Artificial Enamel Lesions in Vivo", Caries Res. 13:80-83 (1979); Hiatt et al., "Root Preparation I. Obduration of Dentinal Tubules in Treatment of Root Hypersensitivity", J. Periodontal. 43: 373-380 (1972); LeGeros et al. "Apatite Calcium Phosphates: Possible Dental Restorative Materials", IADR Abstract No. 1482 (1982); Pickel et al. "The Effect of a Chewing Gum Containing Dicalcium Phosphate on Salivary Calcium and Phosphate", Ala. J. Med. Sci. 2: 286-287 (1965); Zimmerman et al., "The Effect of Remineralization Fluids on Carious Lesions in Vitro", IADR Abstract No. 282 (1979); and U.S. Pats. Nos. 3,679,360 (Rubin) and 4,097,935 (Jarcho).
Generally, the supersaturated solutions or slurries used for remineralization experiments have been prepared from a single form of calcium phosphate. However, these solutions or slurries have been unsatisfactory for a variety of reasons.
In the area of dental cements, the prior art shows an array of compounds. Some cements, however, irritate the pulp and are unsuitable for applications where the cement must come in contact with exposed pulp. Guide to Dental Materials and Devices, 7th Ed. (ADA 1974) p. 49. One solution to this problem is a cement made of materials similar in composition to tooth and bone mineral, since this would not irritate the living tissue.
The use of .beta.-Ca.sub.3 (PO.sub.4).sub.2 was suggested for pulp capping in Driskell et al., "Development of Ceramic and Ceramic Composite Devices for Maxillofacial Application", J. Biomed. Mat. Res. 6:345-361 (1972); and the use of Ca.sub.4 (PO.sub.4).sub.2 O was suggested by Brown and Chow in IADR Abstract No. 120, J. Dent. Res. 54:74 (1975), as a possible pulp capping agent. As described in the latter, Ca.sub.4 (PO.sub.4).sub.2 O hydrolyzes to hydroxyapatite.
Such single calcium phosphate cements are incapable of setting to a hard consistency, however, and suffer from the same drawbacks as single calcium phosphate remineralizers. They cannot maintain a relatively constant pH and do not have sufficient remineralization capacity. Though U.S. Pat. No. 3,913,229 (Driskell et al.) discloses putty-like pastes containing .alpha.-Ca.sub.3 (PO.sub.4).sub.2, .beta.-Ca.sub.3 (PO.sub.4).sub.2, CaHPO.sub.4 and mixtures thereof as pulp capping, root canal, and tooth replanting materials, it is believed that none of these pastes harden into cements. Furthermore, no remineralization properties are disclosed and it is believed that none of these pastes are capable of any substantial remineralization.
Experience with calcium-based implants for the replacement of skeletal tissue has also existed for many years. Most of these implants have been in the form of prefabricated, sintered hydroxyapatite in either granule or block forms. These preparations have several drawbacks, including a limited ability to conform to skeletal defects, particularly in the case of blocks; inadequate structural integrity of granules (which do not bond together), and difficulty in modeling the implant to the shape of missing skeletal tissue with both blocks and granules. The block form of hydroxyapatite provides structural support, but among other complications, must be held in place by mechanical means, which greatly limits its use and its cosmetic results; and it is very difficult to saw a shape such that it fits the patient's individual defect. The granular form produces cosmetically better results, but has a very limited structural stability and is difficult to contain during and after a surgical procedure. In general, all of these products are ceramics, produced by high temperature sintering, and are not individually crystalline, but rather have their crystal boundaries fused together. These ceramic-type materials are in general functionally biologically non-absorbable (having an absorption rate generally not exceeding on the order of 1% per year).
A porous, non-resorbable material based on coral allows intergrowth with bone, but ultimately becomes only approximately 20% bone with the remaining 80% subsisting as scar tissue. HA RESORB made by Osteogen is a form of absorbable hydroxyapatite, but is not a cement. It is granular and not adhesive. HA RESORB is loosely rather than adhesively packed into place. For large uses, it is replaced by bone too quickly. In the dental materials market, HAPSET is a composition of calcium phosphate granules and cementable plaster of Paris (calcium sulfate). This material is not truly a hydroxyapatite and contains too much calcium sulfate for most biological uses. The calcium sulfate component of such a composition is resorbable, but not the calcium phosphate granules.
In sum, the commercially available hydroxyapatite materials are in general not resorbable with accompanying replacement by bone, and are not self-setting (self-hardening) cements.
The patent literature, does, however, describe at least one class of calcium phosphate compositions which are precursors for the formation of hydroxyapatite, and which as slurries, offer good remineralization potential; and, as cements, are biologically compatible, self-setting (self-hardening) and substantially resorbable (biodegradable), with bone replacement, when implanted in contact with living bone tissue. See U.S. Pat. Nos. Re. 33,221 and Re. 33,161 to Brown and Chow, which teach preparation of calcium phosphate remineralization compositions and of a finely crystalline, non-ceramic, gradually resorbable hydroxyapatite cement based on the same calcium phosphate composition. Somewhat similar, and in certain instances potentially identical products are described in U.S. Pat. Nos. 5,053,212, 4,880,610, 5,129,905, 5,047,031, and 5,034,059 to Constantz and others, although the use of non-traditional chemical terminology in the latter patents makes interpretation of them and comparison of them with the prior work of Brown and Chow difficult.
The major components of the calcium phosphate remineralizing slurries, pastes and cements taught in U.S. Pat. Nos. Re. 33,221 and Re. 33,161 are preferably tetracalcium phosphate (Ca.sub.4 (PO.sub.4).sub.2 O), and at least one other sparingly soluble calcium phosphate, preferably dicalcium phosphate anhydrous (CaHPO.sub.4), or dicalcium phosphate dihydrate (CaHPO.sub.4.2H.sub.2 O). These react in an aqueous environment to form hydroxyapatite, the principal mineral in teeth and bones, as the final product. Because of the apatitic nature of the set cement, it is highly compatible with soft and hard tissues. This material, if applied intraoperatively as a paste, subsequently sets to a structurally stable implant composed of microporous hydroxyapatite.